Sound generator



Dec. 19, 1950 G. c. sAvEY ETAL 2,534,833

SOUND GENERATOR Filed Sept. 8, 1948 2 Sheets-Sheet 1 fig. 1

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a/200M C. SEHVY BY Dec@ 39 195@ G. c. sEAvEY ETAL. 2,534,323

SOUND GENERATOR Filed Sept. 8, 1948 2 Sheets-Sheet 2 Patented Dec., 19, 1950 SOUND GENERATOR Gordon C. Seavey, Arlington, and Caperton B. Horsley, Westwood, Mass., assignors to Ultrasonic Corporation, Cambridge, Mass., a corporation of Massachusetts Application September 8, 1948, Serial No. 48,234

12 Claims.

This invention relates to apparatus for generating sound waves and, more particularly to a sound wave generator of the siren type embodying a rotor and stator having cooperating slots and ports.

In sirens of the type herein considered a uid such as air is forced into a rotor having peripheral slots arranged to open and close the ports in a surrounding stator, the stator in turnl communicating with an acoustic horn from which the sound waves are radiated. Inevitably, in such a structure, sound waves will not only be propagated in the stator ports and horn ybut will be directed back into the rotor. If the latter waves were absorbed or led away, the work done to create them would be a power loss of considerable proportions. However, in an ordinary siren these back waves are reected in random fashion within the rotor, and the reflectedv waves generally return to the rotor slots out of phase with the opening of theV stator ports, a condition leading to excessive turbulence at the rotor-stator junction. They might be considered as an irregular A. C. component on the D. C. ow of fluid through the siren. By out of phase in this context, is meant a condition wherein the pressure peak of the reflected wave reaches the rotor slots when the position of the rotor is such that the stator ports are closed or only partiallir opened. If the peak of the reflected wave reaches the rotor slots when the stator ports are fully open then the reilected wave will amplify the sound waves propagated in the stator and horn.

It has been proposed to form a chamber communicating with the input side of the rotor and so dimensioned as to reflect inwardly propagated sound waves back to the rotor-stator junction in phase with the opening of the stator ports, and this proposal directs that the distance from the rotor openings to the reflecting walls of the chamber be equal to an uneven multiple of one quarter of the emitted Wave length. Unfortunately, the problem cannot be answered this simply. Actual experience has shown us that in a siren having such an air chamber communieating with the rotor there will be at certain frequencies a marked increase in acoustic output so that a curve of acoustic power delivered by the generator when plotted against frequency will not be linear or a smooth curve but will eX- hibit dramatic peaks. If thesepeaks occurred at harmonically related frequencies, there would be some support for the uneven quarter wave length theory, but many tests have shown that there is apparently no such harmonic relationship. At this stage of development of the art it appears impossible to calculate in advance the dimensions of a chamber whichr will produce a peak exactly at a given frequency. Where the generator is to be operated at any one of a wide range of frequencies, it is merely a matter of plotting the output as the frequency is varied in order to find the peaks and thereafter operate the generator at one of the peaks thus discovered. On the other hand, where it is essential to operate the generator at a predetermined frequency, it is necessary to resort to other means, and it is the solution of this problem which forms the essence of one aspect of our invention.

A second and related problem resides in the conservation of the sonic energy propagated back into the rotor from the rotor-stator junction. At iiows and pressures requisite to the production of commercially useful sound, a conduit of considerable dimensions isv required for the introduction of compressed duid, such as air, into the rotor. If the back-propagated sound is permitted to reach the conduit, a large portion of the total acoustic power generated will be lost down this conduit. Hence, it is desirable to interpcse a sound baiiie between the rotor and the conduit and so arrange it that the flow of fluid is not impeded but the leakage of sound is efiectively prevented'.

A third source of power loss in a rotor stator combination is the turbulence at the junction of the rotor and stator arising by reason of the fact that ui'd is not emitted radially from the whirlingrotor but, with radial rotor slots, is given a component of motion in the direction of rotation of the rotor. This moment causes turbulence as the iiuid passes into the stator ports. Accordingly, one object of our invention is to provide a rotor having slots so angled as to emit iluid radially as the rotor turns.

We have found that the phase angle of the reilected waves within the rotor may be controlled by providing a chamber communicating with the input side of the rotor and having an end wall mounted for movement toward and from the rotor slots,v thus making it possible to operate the generator at a predetermined frequency and vary theY phase angle of the reflected waves until we arrive at an optimum relationship. Furthermore by placing in the chamber a tubular conduit provided with parallel narrow circumferential slots we provide for the radial emission ofuid into the chamber and thus tothe rotor without letting any significant volume of sound leak into the conduit. Finally, we have discovered that fluid may be radially emitted from the rotor, given radial stator ports, provided each rotor slot axis is angularly oset by a carefully calculated amount from the radius opposite the direction of movement of the rotor. The fluid emitted is thus given a rearward component of motion balancing the forward component resulting from the turning of the rotor.

A sound generator incorporating the three features outlined above has proven much more eflicient in terms of power efiiciency than any device heretofore used or described.

These and other objects and features of the invention will be best understood and appreciated from the following description of a preferred embodiment thereof selected for purposes of illustration and shown in the accompanying drawings in which:

Fig. 1 is a view in longitudinal cross section through a sound generator embodying the features of the invention,

Fig. 2 is a view in end elevation of a portion of the rotor,

Fig. 3 is a view in cross-section through the rotor,

Fig. 4 is a view in transverse cross-section showing diagrammatically the relation of rotor slots and stator ports, and

Figs. 5A, B and C are vector diagrams corresponding to rotor slots of differing angulation.

The organization of the sound generator is best shown in Fig. 1 wherein a flat metal disk I0 is formed integrally with a cylindrical shaft housing I2 containing a centrally disposed bearing race I4. The outer margin of the disk IG is flanged as shown at I6 and shaped to nt an annular stator member I8 bearing on its upper surface a peripheral series of radially arranged triangular teeth defining a series of outwardly diverging ports 22 each of which has a radial axis as clearly shown in Fig. 4. The ports 22 diverge both horizontally and vertically outward to merge smoothly into an annular acoustic horn comprising an upper annulus 24, secured to the flange I6 by a series of bolts 25, and a lower member 28 secured to the stator I8 by means of a series of bolts 28. The design of the stator ports and horn may preferably incorporate the features discussed in our copending application Ser. No. 778,654, filed October 8, 1947, now Patent No. 2,514,129, issued July 4, 1950, for Apparatus for Generating Sound Waves. In brief the stator ports are designed for minimum turbulence and separation and the horn is designed not only as an acoustic amplier but as a passage for uid under pressure.

The upper horn member 24 is reinforced and stiiened by a plurality of webs or ribs 30 located at intervals along its extent, while similar ribs 32 similarly strengthen the lower member 28.

A shaft 34, extending through the housing I2 and supported by the bearing I4, carries at its end a rotor comprising a, flat central hub 36 connected by a thin annulus or web 31 to a peripheral flange 4D, a nut 38 serving to secure the rotor to the shaft 34. The ange 40 is cut to form a series of triangular teeth 44 defining a series of outwardly converging slots 42, as best shown in Figs. 3 and 4. The rotor and stator are concentric, and the radius XY is shown as the axis for the center stator port in Fig. 4, but the medium line or axis of each rotor slot is angularly offset with respect to the radius XY.

The angular offset of the rotor slots is designed to produce radial emission of fluid at the designed speed of the rotor and at the uid pressure to be used in the rotor. The considerations involved are diagrammatically shown in Fig. 5. Fig. 5A is a vector diagram illustrating the condition obtained when a rotor is provided with slots having radial axes. The vector VR represents the angular velocity of the fluid leaving a radial rotor slot, this Vector being imparted to the uid by the angular momentum of the rotor and -being a function of the radius of the rotor and the linear speed of its periphery. The vector VA represents the radial component of the velocity of the air leaving a radial rotor slot, while E represents the resultant of the Vectors VA and VR. It will be seen that the resultant E in Fig. 5A is inclined in the direction of movement of the rotor, and consideration of Fig. 4 will reveal that this resultant will produce excessive turbulence at the rotorstator junction and in the stator ports. In Fig. 5C the vector diagram represents the result of offsetting the axis of the rotor slots an excessive amount. Again VR represents the angular velocity of the air leaving the rotor slot while VA represents the component of the velocity of the air leaving the rotor slot axially. E again represents the resultant and it will be evident that E has a direction producing two results. Considerable driving thrust will be exerted by the fluid leaving the rotor slot, tending to drive the rotor in the direction of its rotation; however, it is equally apparent that considerable turbulence will be created at the rotor-stator junction and in the stator port by reason of the fact that the resultant direction of fluid ow is offset with respect to the axis of the stator ports. The angle 0 represents the amount by which the axis of the rotor slot is offset from the radius. In Fig. 5A this angle is zero and, therefore, does not appear.

In Fig. 5B we have shown a vector diagram showing the result of offsetting the axis of the rotor slots by an amount precisely sufficient to balance the vector VR. VA again represents the velocity of the fluid in the direction of the axis of the rotor slot. The angle 0 represents the amount of onset between the radius XY and the axis of thc slot in the rotor while E represents the resultant of the Vectors VA and VR. In Fig. 5B the resultant E coincides with the radius XY and, therefore, with the axis of the stator port. Consequently the emission of fluid from the rotor of Fig. 5B is radial. There is no thrust at the designed speed of the rotor but there is a minimum of turbulence since the ow entering each stator port is excellent with respect to the stator port. The diagrams and discussion, of course, pertain only to the condition in which the stator ports are entirely open, but it is at this moment that the generation of sound is most eicient.

The value of the angle 0 depends upon the linear speed of the periphery of the rotor, the radius of the rotor, and the pressure of fluid within the rotor. If the generator is to be used at a specific frequency, the design speed of the rotor will be dependent upon the number of slots in the rotor-stator combination. When the diameter of the rotor has been determined and the pressure at which the system will be operated also determined, the angle 0 is easily determined in accordance with the relation:

not be discussed here. The equation sin -l VA as used in the claims is to be construed in the light of the foregoing explanation.

A metal cylinder 50 slightly smaller in diameter than the outside diameter of the rotor is flanged at one end as shown at 52 and bolted to the stator iii by means of a series of bolts 54. At its opposite end the cylinder t is flanged to provide a support for inner metal tube held in place against the end of the cyinder 50 by means of a lock ring The tube 56 is equal in diameter to that of the hub of the rotor, and the inner end of the tube 56 is spaced only a slight amount from the rotor hub. The cylinder 5! and the tube define between them a long annular chamber 53 communicating at one end with the space between the hub 36 and the peripheral flange l0 of the rotor. The tube 55 is provided with a long series of circumferential slots t2 each of which is perpendicular to the longitudinal axis of the tube and relatively long compared to its width. Furthermore the slots are spaced apart by an amount comparatively large with respect to the width of the slots. It is contemplated that air or other fluid under pressure will be introduced into the tube 56 and emitted radially into the chamber 5l through the slots However, the arrangement of the slots is such that very little sound will pass from. the chamber 5l into the tube 55 and the arrangement of slots is, therefore, to be considered a sound baffle or, with respect to the tube 5G, a muffler.

Adjacent the outer end of the chamber 5l there is mounted a thick metal annulus 5t provided on opposite sides with set screws 65 which work in narrow slots 08 cut into the side of the cylinder 50. The position of the annulus 64 may be adjusted longitudinally along the chamber 5l by loosening the set screws t6 and moving the annulus back and forth. At its outer end the cylinder 50 is provided with a rubber collar l0 and a coupling support lli bolted to the end of the cylinder 50, the arrangement providing for the connection of a pipe 'l2 shown in dotted lines and leading to a source of compressed air or other pressure fluid (not shown).

The shaft 313 may be driven conventionally by a motor, turbine or the like to turn the rotor at desired speed. The number of slots in the rotor is equal to the number of ports in the stator and their arrangement is such that the stator ports are opened and closed by the rotor in unison. Consequently, the frequency of the sound generated is the product of the number of rotor slots and the speed of rotation. For example, at 2,000 R. P. M. an 30 slot rotor will generate sound at the rate of 160,000 cycles per minute or 2,666 cycles per second. The uid which is introduced to the rotor under pressure leaves it as the stator ports are opened in sharp bursts which are sound waves of complex form. As previously stated, 50% of the acoustic power produced is propagated through the stator ports and the horn while the other 50% is propagated back through the rotor and into the cham-ber 5l.v It will be found that for any given position of the annulus 64 there will be sharp peaks in the curve of acoustic output as the frequency varies. For example, measurements made on a generator of a type similar to that shown in the drawings revealed peaks of acoustic output at the following frequencies:

1,750 C. P. S. 3,150 C. P. S. 6,900 C. P. S. 11,000 C. P, S.

and a slightly diiferent form showed peaks at 1,750 C. P. S. 4,100 C. P. S.

Obviously, these frequencies are not harmonically related and the phenomena involved are not as yet suiciently understood to make it possible to predict in advance the frequencies at which the peak outputs will be obtained. However, if it is desired to operate a generator at a cert-ain frequency, it is of course desirable to so construct the generator that the selected frequency will be one at which the output is highest. This may be accomplished by means of the annulus since the physical dimensions of the chamber apparently determine the frequencies at which the peaks will be obtained. By turning the shaft Sli at the proper speed to produce the desired frequency and then varying the position of the annulus S while measuring the output of the generator, it is a comparatively simple matter to attain an optimum condition.

It is important to note that the baifzing slots 62 play an important part in the eciency with Which the generator operates. If sound were permitted to enter the tube 56, it would pass down the conduit and be lost. If the passage of sound into the tube 5S were unimpeded, the power thus lost might approximate 50% of the total amount generated. In other words, the slots 62 conserve the sound by containing it within the chamber 5? and within the rotor. The function of the chamber is to reflect the back-propagated waves to the rotor-stator junction in proper phase relation with the opening of the stator ports, and this function is obtained, for any given frequency, by the adjustment of the annulus 6d. In other words, by first preventing the escape of backpropagated sound and then properly reflecting it to the rotor-stator junction, we have succeeded in materially increasing the eilciency with which sound may be generated. rEhe angulation of the rotor slots to provide radial emission of the iluid from the rotor into the radial stator ports still further increases the efficiency with which sound is generated by diminishing the turbulence at the rotor-stator junction- Having thus disclosed our invention what we claimy as new and desire to secure by Letters Patent of the United States is:

1. A sound generator comprising a ported stator, a slotted rotor mounted to turn within said stator, walls forming a chamber communieating with the slots of the rotor through the interior thereof, a conduit, a sound barde connecting the conduit to the chamber, and a bottom wall adjustably mounted within the chamber for varyingy the effective distance travelled by sound waves propagated in the chamber by the interaction of the rotor and stator whereby fluid may be passed sequentially through the conduit, the chamber, the rotor slots and the stator ports.

2. A sound generator comprising a ported stator, a slotted rotor mounted adjacent the stator, walls forming a chamber on the input side of the rotor, an input conduit communicating with said chamber, and an end wall adjustably mounted in said chamberto vary the distance travelled by sound waves propagated in the chamber by the interaction of the rotor and stator.

3. A sound generator comprising a rotor having a chamber communicating with a, series of peripheral slots, a stator surrounding the rotor and having a series of ports disposed in position to be opened and closed in unison as the rotor turns, a slotted conduit disposed co-aXially with the rotor, walls forming an annular chamber surrounding the conduit and communicating with the rotor chamber, and an adjustably mounted end wall closing said annular chamber, said wall being movable to reect sound waves of predetermined range of frequencies to the rotor slots in phase with the opening of the stator ports by the rotor.

4. A sound generator comprising a slotted rotor, a ported stator surrounding the rotor, a tubular conduit disposed co-axially with the rotor and having a plurality of transverse slots each relatively narrow compared to its length and spaced from adjoining slots by an amount relatively greater than its width, and walls forming a chamber surrounding said conduit and communicating with said rotor.

5. A sound generator comprising an annular ported stator, a rotor comprising a nat hub and an annular slotted Wall spaced from said hub, said rotor being mounted within said stator and co-axial therewith, a tubular conduit co-aXial with said rotor and having substantially the same diameter as the hub thereof, said conduit terminating closely adjacent one surface of said hub, said conduit being provided with a series of long narrow circumferential slots, each slot being spaced from adjacent slots by an amount exceeding the width of the slot, and walls forming a chamber surrounding said slotted conduit and communicating with the slots of the rotor.

6. A sound generator comprising an annular ported stator, a rotor mounted Within said stator and having a flat hub and an annular slotted outer Wall spaced from said hub, Walls forming a tubular chamber communicating with the slots of the rotor through the space between the hub and the outer wall thereof, a conduit entering said chamber, and a sound baffle connected to said conduit within said chamber in position to form the sole passage for fluid passing through the conduit and into said chamber.

7. A sound generator comprising a ported stator, a slotted rotor associated with said stator, walls forming a chamber communicating with the input side of said rotor, and a tubular conduit entering said chamber, said conduit being provided with the series of parallel circumferential slots, each slot being long in comparison to its width and spaced adjacent slots by an amount substantially greater than the width of the slot.

8. A sound generator comprising an annular stator having a series of outwardly diverging radial ports, a rotor mounted within said stator and having an annular chamber, the outer wall of said chamber being provided with a series of slots communicating with the stator ports, each slot converging outwardly and the axis of each 8 slot being offset from the radius opposite the drection of rotation of the rotor by an angle 0 derived from the expression,

where VR is a function of the radius and R. P. M. of the rotor and VA is a function of the pressure of fluid and configuration of the rotor slots.

9. A sound generator comprising an annular ported stator, a slotted rotor mounted within said stator, a cylindrical conduit terminating closely adjacent said rotor and co-axial therewith, walls forming an annular chamber surrounding said conduit and communicating with the rotor slots, an annui-us mounted in said annular chamber, said annulus completely surrounding the conduit and substantially filling its entire radial extent, and means for adjusting said annulus longitudinally along said chamber.

10. A sound generator comprising a ported stator, a slotted rotor mounted adjacent said stator, walls forming a chamber communicating with the input side of said rotor, a conduit entering said chamber, a mufller interconnecting said chamber and substantially lling the cross-sectional area of said chamber, and said conduit, a wall mounted within said chamber and mea-ns for adjusting said wall toward and from the rotor.

l1. A sound generator comprising an annular stator having a series of outwardly dverging radial ports, a rotor mounted within said stator and having an annular chamber, the outer Wall oi said chamber being provided with a series of slots communicating with the stator ports, the axis of each slot being angularly odset from the radius by an amount substantially satisfying the following relation:

sin Q VA where VR is a function of the radius and R. P. M. of to rotor and VA is a function of the pressure of fluid and the configuration of the rotor slots.

12. A sound generator comprising a radially ported stator, a rotor mounted adjacent the stator and having a series of slots communicating with the stator ports, the axis of the stator ports by an angle 0 substantially satisfying the relation:

where VR is a function of the radius and R. P. M. of the rotor and VA is a function of the pressure of fluid and the configuration of the rotor slots.

GORDON C. SEAVEY. CAPERTON B. HORSLEY.

REFERENCES CITED rThe following references are of record in the ile of this patent:

UNITED STATES PATENTS Number Name Date 780,674 Leech Jan. 24, 1905 916,707 Hoenow Mar. 30, 1909 965,897 Haskell Aug. 2, 1910 1,179,379 Conner Apr. 1l, 1916 1,278,095 Beckwith Sept. 10, 1918 1,585,219 Webb May 18, 1926 1,735,864 Hutchison Nov. 19, 1929 2,019,596 Broden Nov. 6, 1935 2,434,175 Ozols Jan. 6, 1948 

